The invention relates to a photovoltaic solar cell and to a method for producing a photovoltaic solar cell.
Photovoltaic solar cells typically consist of a semiconductor structure having a base region and an emitter region, wherein the semiconductor structure is typically substantially formed by a semiconductor substrate, such as a silicon substrate, for example. Light is coupled into the semiconductor structure typically via the front side of the solar cell, such that generation of electron-hole pairs takes place after absorption of the coupled-in light in the solar cell. A pn junction forms between base and emitter region, the generated charge carrier pairs being separated at said junction. Furthermore, a solar cell comprises a metallic emitter contact and also a metallic base contact, which are respectively electrically conductively connected to the emitter and to the base. Via these metallic contacts, the charge carriers separated at the pn junction can be conducted away and thus fed to an external electric circuit or an adjacent solar cell in the case of module interconnection.
Various solar cell structures are known, wherein the present invention relates to those solar cell structures in which both electrical contacts of the solar cell are arranged on the rear side, wherein electrical contact can be made with the base of the solar cell via a metallic base contact structure arranged on the rear side, and electrical contact can be made with the emitter of the solar cell via a metallic rear-side contact structure arranged on the rear side. This is in contrast to standard solar cells, in which typically the metallic emitter contact is situated on the front side and the metallic base contact is situated on the rear side of the solar cell.
In this case, the invention relates to a specific configuration of a solar cell with which contact can be made on the rear side, the metal wrap-through solar cell (MWT solar cell). This solar cell, known from EP 985233 and van Kerschaver et al. “A novel silicon solar cell structure with both external polarity contacts on the back surface” Proceedings of the 2nd World Conference on Photovoltaic Energy Conversion, Vienna, Austria, 1998, indeed has a metallic front-side contact structure arranged at the front side of the solar cell designed for coupling in light, said front-side contact structure being electrically conductively connected to the emitter region. However, the solar cell furthermore has a multiplicity of cutouts extending from the front side to the rear side in the semiconductor substrate, metallic feedthrough structures penetrating through said cutouts and said cutouts being electrically conductively connected on the rear side to one or more metallic rear-side contact structures, such that electrical contact can be made with the emitter region on the rear side via the rear-side contact structure, the feedthrough structure and the front-side contact structure.
The MWT structure has the advantage that the charge carriers are collected from the emitter at the front side via the front-side contact structure and, consequently, no ohmic losses arise as a result of possible charge carrier transport within the semiconductor substrate from the front side to the rear side with regard to the emitter region. Furthermore, the capability of making contact both with the base region and with the emitter region on the rear side results in a simpler interconnection of the MWT solar cells in the module compared with standard solar cells.
What is disadvantageous about the MWT structure is that, compared with standard solar cells, it is necessary to produce additional structures such as, for example, the cutouts and the metallic feedthrough structures through the cutouts, thus resulting in a higher complexity and hence higher costs compared with the production of standard solar cells. Moreover, in particular in the case of inaccurate processing on the walls of the cutouts and also in the region in which the rear-side contact structures cover the rear side of the solar cell, there are risks concerning the formation of additional loss mechanisms; in particular, short-circuit currents can occur if the rear-side contact structure through a fault penetrates into the base region of the semiconductor substrate (so-called “spiking”), as a result of which the efficiency of the solar cell is considerably reduced.
For this reason, EP 0 985 233 proposes leading the emitter through the cutouts and, at the rear side, at least beyond the regions covered by the rear-side contact structure, such that the rear-side contact structure, serving for making contact with the emitter externally, covers no region of the semiconductor substrate having the base doping.
However, this requires complex processing and a plurality of cost-intensive masking steps.